Nowadays, as digital mobile technology develops rapidly, femtocells are already being introduced into homes. A femtocell based on a Digital Subscriber Line (DSL) system is greatly in need of a low-cost and accurate clock synchronization solution.
Typical DSL systems have a clock synchronization method which directly depends upon a network reference clock. The method is described as follows. It is assumed that a working clock of a transmission (TX) unit on a central office (CO) side is C1 and C1 is directly obtained through frequency multiplication or phase-locked frequency multiplication of the network reference clock CLK2, the working clock C1 is transferred to a Customer Premises Equipment (CPE) side through a physical signal. C1′ is a working clock restored by the CPE side from the physical signal. C1′ is a mirror of C1. Therefore, C1′ is synchronous with C1. Moreover, C1 is further synchronous with the network reference clock CLK2, so C1′ is further synchronous with the network reference clock CLK2. CLK2′ is frequency-divided from C1′. The clock frequency synchronization between CLK2′ and CLK2 may be achieved by matching factors of frequency multiplication on the CO side and the frequency division on the CPE side.
However, the working clock of DSL systems in the prior art is a network reference clock. The working reliability of the system depends upon the reliability of the network reference clock. If a failure occurs on the network reference clock, the normal operation of the whole DSL system is affected.